There are no perfect solutions to the this. To maintain the full quality of the images in Aperture, your best bet is going to be exporting the Master files from Aperture, then re-importing them into iPhoto.

If you've done extensive edits to your images, you will also want to export versions of those files in the highest quality possible, and import those versions into your iPhoto library.

Another possibility—that I haven't fully explored—would be to export your Aperture library as a new library that uses referenced images (as opposed to managed) so that those images exist in an organized folder hierarchy on your hard drive. You can then import those images and keep some of your Aperture organizational structure. (This is unexplored by me because I've never used the referenced image option.)